Bipolar plate for fuel cell

ABSTRACT

A bipolar plate for a fuel cell includes an anode plate and a cathode plate. The anode plate has hydrogen flow channels on a first side of the anode plate and coolant channels on a second side of the anode plate. The cathode plate has a first side disposed against the second side of the anode plate to cover the coolant channels and has a second side defining a recessed pocket configured to receive a stream of air. A flow guide is disposed in the pocket such that an inlet manifold is formed along a first edge of the flow guide and an outlet manifold is formed along a second edge of the flow guide. The flow guide defines channels extending from the inlet manifold to the outlet manifold. A plurality of openings is defined by through the flow guide.

TECHNICAL FIELD

The present disclosure relates to fuel cells for automotive vehicles andmore specifically to flow-field designs of bipolar plates.

BACKGROUND

The hydrogen fuel cell, and in particular the proton exchange membranefuel cell (PEMFC), is one potential power source for automobiles andstationary applications. The reaction in a PEMFC involves hydrogenmolecules splitting into hydrogen ions and electrons at the anode, whileprotons re-combine with oxygen and electrons to form water and releaseheat at the cathode. Typically, a proton exchange membrane is used as aproton conductor in a PEMFC. A catalyst layer containing, for example,platinum and/or a platinum alloy is used to catalyze the electrodereactions. A gas diffusion layer, which may include a microporous layerand a gas diffusion backing layer, is used to transport reactant gasesand electrons as well as remove product water and heat.

SUMMARY

According to one embodiment, a bipolar plate for a Fuel cell includes ananode plate and a cathode plate. The anode plate has hydrogen flowchannels on a first side of the anode plate and coolant channels on asecond side of the anode plate. The cathode plate has a first sidedisposed against the second side of the anode plate to cover the coolantchannels and has a second side defining a recessed pocket configured toreceive a stream of air. A flow guide is disposed in the pocket suchthat an inlet manifold is formed along a first edge of the flow guideand an outlet manifold is formed along a second edge of the flow guide.The flow guide defines channels extending from the inlet manifold to theoutlet manifold. A plurality of openings is defined by through the flowguide.

According to another embodiment, a bipolar plate for a fuel cellincludes an anode side having hydrogen channels and a cathode sidedefining a recessed pocket. Coolant channels are disposed between theanode side and the cathode side. At least one air port is in fluidcommunication with the pocket, A flow guide is disposed in the pocketsuch that an inlet manifold is formed along a first edge of the flowguide and an outlet manifold is formed along a second edge of the flowguide. The flow guide defines channels extending from the inlet manifoldto the outlet manifold. A plurality of openings is defined through theflow guide.

According to yet another embodiment, a fuel cell includes a plurality ofunit cells disposed in a stack. Each unit cell includes a membraneelectrode assembly (MEA) having an anode and a cathode, a bipolar plate,and a. flow guide. The bipolar plate has a cathode side defining arecessed pocket in fluid communication with an air port, an anode side,and coolant channels between the cathode and anode sides. The bipolarplate is disposed against the MEA such that the cathode is disposed overthe pocket. The flow guide is disposed in the pocket with a front sidefacing the MEA and a back side facing a bottom of the pocket. The flowguide includes a plurality of embossments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a fuel-cell vehicle.

FIG. 2 is an exploded view of a proton exchange membrane fuel cellstack.

FIG. 3 is a side cross-sectional view of a portion of a fuel cell stackshowing two unit cells.

FIG. 4a is a front view of an anode plate showing the hydrogen flowside.

FIG. 4b is a back. view of an anode plate showing the coolant flow side.

FIG. 5a is a back view of a cathode plate showing the oxidant flow side.

FIG. 5b is a front view of a cathode plate showing the coolant flowside.

FIG. 6 is a front view of the cathode plate with a flow guide installed.

FIG. 7 is a perspective view of a flow guide according to an embodiment.

FIG. 8 is a cross-sectional view of the cathode plate.

FIG. 9 is a perspective view of a flow guide according to anotherembodiment.

FIG. 10 is a front view of another cathode plate.

FIG. 11 is a side view of an embossment of a flow guide of the cathodeplate of FIG. 10.

FIG. 12 is a detail view of an embossment having holes.

FIG. 13 is a front view of a flow guide according to another embodiment.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. it is to beunderstood, however, that the disclosed embodiments are merely examplesand other embodiments can take various and alternative forms. Thefigures are not necessarily to scale; some features could be exaggeratedor minimized to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting but merely as a representative basis forteaching one skilled in the art to variously employ the presentinvention. As those of ordinary skill. in the art will understand,various features illustrated and described with reference to any one ofthe figures can be combined with features illustrated in one or moreother figures to produce embodiments that are not explicitly illustratedor described. The combinations of features illustrated providerepresentative embodiments for typical applications. Variouscombinations and modifications of the features consistent with theteachings of this disclosure, however, could be desired for particularapplications or implementations.

PEMFC are a popular fuel cell choice for automotive vehicles. The PEMFCgenerally includes a proton exchange membrane (PEM). The anode and thecathode typically include finely divided catalytic particles, usuallyplatinum, supported on carbon particles and mixed with an ionomer. Thecatalytic mixture is deposited on opposing sides of the membrane. Thecombination of the anode-catalytic mixture, the cathode-catalyticmixture, and the PEM form a coated catalyst membrane electrode (CCM). Inorder to facilitate the transport of reactant gases to and remove theexcessive water and heat from the catalytic mixture, a gas diffusionlayer (GDL), which may include a microporous layer and a.carbon-fiber-based gas diffusion backing layer, may be applied on eitherside of the CCM to form a membrane electrode assembly (MEA). GDLs alsoprovide mechanical support for the soft goods including the PEM andcatalytic mixtures.

MEAs are sandwiched between bipolar plates to form. unit cells. Thebipolar plates typically include an anode side and a cathode side. Anodefuel flow channels are provided on the anode side of the bipolar platesthat allow the anode gas to flow to the anode side of the MEA. Cathodeoxidant flow channels are provided on the cathode side of the bipolarplates that allow the cathode gas to flow to the cathode side of theMEA. Coolant channels may be disposed between the anode and cathodesides of the bipolar plates to thermally regulate the fuel cell.

Several unit cells are typically combined in a fuel cell. stack togenerate the desired power. For example, the stack may includetwo-hundred or more unit cells arranged in series. The fuel cell stackreceives a cathode reacting gas, typically a flow of air forced throughthe stack by a compressor. Not all the oxygen is consumed by the stackand sonic of the air is output as a cathode exhaust gas that may includewater as a stack byproduct. The fuel cell stack also receives an anodehydrogen. reacting gas that flows into the anode side of the stack.

Referring to FIG. 1, a vehicle 10 includes a fuel cell 20 for providingelectrical power to at least one electric machine 12. The vehicle 10 mayalso include a traction battery 14 electrically connected to the fuelcell 20 and the electric machine 12. The electric machine 12 isconnected to the driven wheels 16 via a drivetrain 18. During operationof the vehicle 10, hydrogen fuel and air are fed into the fuel cell 20creating electrical power. The electric machine 12 receives theelectrical power as an input, and outputs torque for driving the wheels16 to propel the vehicle 10.

Referring to FIG. 2, an example fuel cell 20 includes two unit cells 22,24 stacked together. The two-cell stack is merely an example and thefuel cell 20 may include dozens or hundreds of stacked unit cells. Thefirst unit cell 22 includes an MEA 26 sandwiched between a first endplate 28 and a bipolar plate 30. The MEA 26 is comprised of a pluralityof different layers including a PEM 32, a pair of gas diffusion layers(GDL) 34 and a pair of catalyst layers 36. The endplate 28 includes ananode side 38 defining a plurality of flow paths 40 for the hydrogenfuel. The bipolar plate 30 includes a cathode side 42 defining aplurality of flow paths 44 for air and an anode side 46 defining aplurality of flow paths 48 for hydrogen fuel for the second unit cell24. A second MEA 50 is sandwiched between the bipolar plate 30 and alast endplate 52. The last endplate 52 includes a cathode side 54defining a. plurality of flow paths 56 for air. The coolant channels 58,60, 62 are configured to circulate coolant, such as ethylene glycol.

Referring to FIG. 3, a fuel cell 70 includes a. repeating pattern ofunit cells 72 that are linearly arranged a stack 71. Most unit cellincludes a bipolar plate 74 and an MEA 76. The unit cells at the ends ofthe stack may include end plates rather than bipolar plates. FIG. 3illustrates two unit cells, but in practice, the fuel cell stack 70 mayinclude dozens or hundreds of unit cells 72.

Each bipolar plate 74 may include one or more subassemblies of plates.In the illustrated embodiment, each bipolar plate 74 includes an anodeplate 78 and a cathode plate 80. But, in other embodiments, the anodeplate 78 and the cathode plate 80 may be portions of a singularstructure. As illustrated, the unit cells 70 are the anode plate 78adjacent to the cathode plate 80, then the MEA. 76 adjacent to thecathode plate 80, repeat.

Referring to FIGS. 4A and 4B, the anode plate 76 may include a frontside 86 and a hack side 88. The front side 86 is disposed against theMEA 76 and the hack side 88 is disposed against the cathode plate 80. Acoolant flow field 90 may he provided on the hack side 88 and a hydrogenflow field 92 may he provided on the front side 86. The anode plate 78includes an outer frame 94 that is outside of the reactant area of thefuel cell, whereas the coolant and hydrogen flow fields 90, 92 arewithin the reactant area.

The outer frame 94 may include opposing sides 96 and 98 that define aplurality of ports for the coolant, the hydrogen, and the air. Forexample, a hydrogen supply port 100 is defined in the side 96, and ahydrogen return port 102 is defined in the side 98. The hydrogen flowfield 92 receives hydrogen from the supply port 1.00, circulates thehydrogen across the MEA 76, and returns excess hydrogen to the returnport 102. The hydrogen flow field 92 may include a plurality of channelsoriented to extend from the side 96 to the side 98. The channels may bedefined in the anode plate or may be formed on a flow guide. In theillustrated embodiment, the front side 86 defines a front pocket 104configured to receive a flow guide 106 therein. The flow guide 106defines a plurality of channels 108. The channels 108 may be linear andextend from the side 96 towards the side 98. An. inlet passage 110supplies hydrogen to the coolant flow field 92 and an outlet passage 112returns hydrogen to the return port 102. The flow guide 106 may includea portion disposed against the MEA to provide an electrical connectionbetween the anode plate 76 and the MEA.

The outer frame 94 may also define a. coolant inlet port 116 disposed onside 96 and a coolant return port 118 disposed on the side 98. Thecoolant flow field 90 receives coolant from the supply port 116,circulate the coolant across the bipolar plate, and returns the coolantto the return port 118. The coolant flow field 90 may include aplurality of channels configured to convey the coolant. The coolant maybe an ethylene glycol mixture or other coolant formula. in theillustrated embodiment, the back side 88 defines a back pocket 120configured to receive a coolant flow guide 122 therein. The coolant flowguide 122 may define a plurality of channels 124 that extend from theinlet side to the outlet side of the pocket 120. An inlet passage 126and an outlet passage 128 fluidly connect the supply and return ports116, 118 with the coolant flow field 90.

The anode plate 78 and the flow guides 106 and 122 may be formed ofgraphite composite material or metal. In some embodiments, the anodeplate 78 and the flow guides 106 and 122 are formed of the same type ofmaterial. For example, anode plate 78 and the flow guides 106 and 122may be formed of graphite, or the anode plate 78 and the flow guides 106and 122 may be formed of an electrically conductive composite. Thecomposite may include carbon power and a binder.

Referring to FIGS. 5A and 5B, the cathode plate 80 may include frontside 140 and back side 142. When the bipolar plate 24 is assembled theback side 142 of the cathode plate 80 is disposed against the back side88 of the anode plate 78. The back side 142 of the cathode plate 80 maybe flat and planar. The back side 142 covers over the coolant flow field90 and cooperates with the anode plate 78 to fully define the coolantflow field 90, which is located in the middle of the bipolar plate 74.

The front side 140 of the cathode plate 80 is disposed against thecathode side of the MBA 76. The cathode plate 80 includes an outer frame144 that is outside of the reactant area of the fuel cell. The outerframe 144 may include opposing sides 146 and 148 that define a pluralityof ports for the coolant, the hydrogen, and the air. The ports of thecathode plate 80 align with the ports of the anode plate 78. The MBA 76also includes coolant, hydrogen, and air ports that are aligned with theports of the cathode plate and the anode plate. The ports of the unitcells 72 are all aligned to create coolant, hydrogen, and air headersthat extend the along the length of the stack. Seals may be providedbetween the frames of the bipolar plates to prevent leaking of theheaders. The seals may be elastomeric or silicone. In one embodiment,the seals may be Polytetrafluoroethylene (PTFT).

For example, a hydrogen supply port 150 is defined in the side 148, anda hydrogen return port 152 is defined in the side 148. A coolant supplyport 154 is defined in the side 148, and a coolant return port 156 isdefined in the side 146. An air supply port 158 is defined in the side146 and an air return port 160 is defined in the side 148.

An air flow field 162 is located on the front side 140 of the cathodeplate 80 and is configured to circulate air over the reactant area ofthe cathode side of the MEA 76. The air flow field 162 may be locatedwithin a pocket 164 recessed into the front side 140 of the cathodeplate. The pocket 164 may include a bottom 166, opposing top and bottomsidewalls 168, 170 and opposing left and right sidewalls 172, 174. (Theterms top, bottom, right, left, etc., are for ease of description and donot limit the embodiments of this disclosure to any particularorientation.) The front side 140 of the cathode plate 80 is disposedagainst the cathode side of the MEA 76 so that the MEA 76 covers thepocket to enclose the air flow field 162. The air supplied by the airflow field 162 forms part of the chemical reaction of the fuel cell. Theoxygen atoms in the air combine with the hydrogen ions to form waterthat is carried away by the airstream flowing through the air flow field162.

The pocket 164 is in fluid communication with the air supply port 158and the air return port 160. For example, an inlet passage 176 extendsfrom the air supply port 158 and through the wall 172, and an outletpassage 178 extends from the air return port 160 through the wall 174.In the illustrated embodiment, air is supplied to the pocket 164 nearthe top and exits near the bottom, but this could be reversed. The depthof the pocket 164, measured from the outer surface of the frame 144 tothe bottom 166, may be varied with the thickness of the flow guide. Thedepth may be set so that lands of the flow guide contact the MEA. Thearea of the pocket 164 may approximate the reactive area of the MEA 76.A flow guide (not shown) having air channels may be disposed within thepocket. This will be described in detail below.

The cathode plate 80 may be formed of graphite or composite materials.In some embodiments the anode plate 78 and the cathode plate 80 areformed of the same material. The cathode plate 80 may also be formed ofthe same material as the flow guides 106 and 122. In one embodiment, theanode plate 78, the cathode plate 80, and the flow guides 106, 122 areformed of graphite. Or, the anode plate 78, the cathode plate 80, andthe flow guides 106, 122 may be formed of composite.

FIGS. 6 and 7 illustrate an example flow guide 200 disposed within thepocket 164 of the cathode plate 80. The flow guide 200 includes an innerside 190 facing the bottom 166 of the pocket 164 and an outer side 192facing the MEA 76 when the fuel cell 20 is assembled. The flow guide 200defines channeling 198 configured to circulate the air. The flow guide200 is designed to circulate air on both the inner side 190 and theouter side 192. In the illustrated embodiment, the channeling 198 islinear and is oriented to circulate air from the top towards the bottom.The flow guide 200 may include a top edge 202, a bottom edge 204, andopposing side edges 206 and 208. The width of the flow guide 200(distance between edges 202 and 204) may be less than the width of thepocket (distance between sidewalk 168, 170) to form an upper gap and alower gap. The upper gap serves as an inlet manifold 210 that receivesair from the inlet passage 176. The lower gap serves as an outletmanifold 212 that collects air and water prior to circulation to theoutlet passage 178. During operation of the fuel cell 70, high pressuredevelops in the inlet manifold 210 causing the air to circulate downthrough the channeling 198 and subsequently into the outlet manifold212. The length of the flow guide 200 (between edges 206 and 208) mayapproximate the length of the pocket 164.

The flow guide 200 may be formed as a corrugated metal plate. The metalplate may be made of a corrosion-resistance metal such as stainlesssteel, titanium, or aluminum alloy. The metal plate may have a corrosionresistant coating. That is, the flow guide 200 may be formed of amaterial that is different than the material of the cathode plate 80.The metal plate may have a thickness of 0.10 to 0.20 millimeters (mm)prior to forming, and once formed, may have a final thickness between0.2 mm to 1.0 mm. The corrugations may be rectangular (as shown), wavy,or trapezoidal. The corrugations define the channeling 198. Thechanneling 198 may be on both the inner side 190 and on the outer side192 of the flow guide 200. The channeling 198 may define concavechannels 214 extending across the outer side 192 and convex channels 216extending across the inner side 190. The channels 214, 216 may eachinclude opposing side walls 218 and 220 that are interconnected byeither an inner land 222 or and outer land 224 depending upon thechannels being concave or convex. The channeling 198 may besubstantially perpendicular to the first and second edges 202, 204.Substantially perpendicular means within plus or minus 3 degrees ofperpendicular.

Referring to FIG. 8, the flow guide 200 has a thickness (T) that ismeasured from the inner-most surface to the outer-most surface, e.g.,the distance from the inner lands 222 to the outer lands 224. Since theflow guide 200 is formed of metal, the flow guide 200 has resiliency andcan be urged from a resting state to a compressed state. In the restingstate, the thickness of the now guide 200 is greater than the depth ofthe pocket 164 so that the flow guide projects outwardly from thecathode plate 80 by a distance (D). The distance D may be between 0.05mm and 0.3 mm. This causes the flow guide 200 to be compressed betweenthe MEA 76 and the bottom 166 of the pocket to ensure sufficient contactbetween the gas diffusion layer of the MEA and the outer lands 224. Inother embodiments, the thickness (T) of the flow guide 200 mayapproximate or be less than the depth of the pocket 164.

Referring back to FIG. 7, the flow guide 200 may define a plurality ofopenings 226 connecting the convex channels 216 and the concave channels214 in fluid communication so that air and/or water can pass through theflow guide 200. The inclusion of the openings 226, in conjunction withthe channels on the inner side 190 and outer side 192, creates what maybe referred to as a 3D flow field as the water is able to not only flowacross the front and the back but also through the flow guide 200. The3D nature of the flow field 162 created by the flow guide 200 increasesuniformity of the air and water flow, which may reduce the probabilityof water blockage. During operation of the fuel cell, water tends tocondense more against the bottom 166 of the pocket as it is adjacent tothe coolant flow field 90. The openings 226 promote water to travel tothe dryer outer side 192 to more evenly distribute the water within theflow field 162 and help reduce water blockages.

In the illustrated embodiment of FIG. 7, the openings 226 are defined inthe sidewalk 218. But, in other embodiments, the openings may be definedin the inner lands 222, the sidewalls, 220, and/or in. the outer lands224. In other embodiments, openings may be defined in both lands andsidewalls. For example, the openings may be defined in the outer lands224 and one or more of the sidewalk 218, 220 or in the inner lands 224and one or more of the sidewalls 218, 220. The openings may be holes,slits, slots or other suitable shape. In some embodiments, the openings226 may be uniform and of the same type, but, in other embodiments,multiple different types of openings may be provided on the flow guide.

FIG. 9 illustrates another flow guide 250 that may be used in thecathode plate 80. The flow guide 250 may also be a corrugated metal flowguide having convex and concave channels 252, 254 that extend betweenthe opposing edges. The flow guide 250 includes openings in the form ofslits 256. The slits 256 may extend through the outer lands and/or thesidewalls of the channels. The slits may have a width of between 0.05 mmto 0.2 mm and a length of 3.0 mm to 20.0 mm. The slits 256 may be spacedapart by 0.2 mm to 1.0 mm. The slits may be angled at an oblique anglerelative to the direction of the channels. For example, the slits 256may be angled between 10° to 45° relative to of the centerlines of thechannels.

The slits 256 may be arranged in sections in which the slits extend in acommon direction. The illustrated flow guide 250 includes at least afirst section 260 in which the slits have a first orientation and asecond section 262 in which the slits have a second orientation. Thefirst and second orientations may be mirrored over a line 264 as shownin FIG. 9. The orientations and slit sizes may vary with the area ofcathode flow field to tune the systems for optimal performance.

FIG. 10 shows another cathode plate 350 that may be used in a modifiedversion of the bipolar plate 74. The cathode plate 350 may include afront side 352 and a back side (not visible). The back side of thecathode plate 350 is disposed against the back side of the anode plateas described above. The front side 352 of the cathode plate 350 isdisposed against the cathode side of the MEA. The cathode plate 350includes an outer frame 354 that is outside of the reactant area of theMEA. The outer frame 354 may include opposing sides 356 and 358 thatdefine a plurality of ports for the coolant and the hydrogen. A top 360and a bottom 362 of the frame 354 may each define a plurality of airports with the air supply ports 364 being on the bottom 362 and the airreturn ports 366 being at the top 360. Alternatively, the air portscould be disposed on the right and left sides 356, 358 like the cathodeplate 360. The ports of the cathode plate 350 align with the ports ofthe anode plate and the MEA to create coolant, hydrogen, and air headersthat extend the along the length of the fuel-cell stack.

An air flow field 368 is located on the front side 352 of the cathodeplate 350 and is configured to circulate air over the reactant area ofthe cathode side of the MEA. The airflow field 368 may be located withina pocket 370 recessed into the front side of the plate. The pocket 370may be the same or similar to the pocket 364.

Referring to FIGS. 10 and 11, a flow guide 372 may be disposed in thepocket 370 such that an inlet manifold 374 and an outlet manifold 376are formed. During operation, air flows over a front side 378 and a backside 380 of the flow guide 372 similar to above. Rather than havingchanneling, the flow guide 372 includes a plurality of embossments 382raised from the front side 378 and recessed into the back side 380. Eachof the embossments 382 creates a projection 384 raised from the frontside 378 and a depression 386 recessed into the back side 380. Each ofthe embossments 382 also includes a top 388 and a plurality of sidewalk390 extending between the front side 378 and the top 388.

The flow guide 372 may have a plate thickness (T) between 0.1 mm to 0.3mm, and the embossments may have a height (H) between 0.2 mm to 2.0 mm.The shape of the embossments 382 may be rectangular (as shown), wedged,circular, elliptical, triangular, trapezoidal, arrow, hexagonal, or anyother polygonal shape. The embossments 382 are arranged in a pattern tooptimize the flow rate and uniformity of the flow field 368. Thisdisclosure contemplates many different patterns of the embossments 382.For example, the embossments 382 may be arranged in rows to roughlycreate linear channels extending from the inlet manifold 374 to theoutlet manifold 376. In the illustrated embodiment, the embossments 382are arranged in angled rows 392 to roughly define angled channels 394.The embossments in adjacent rows are offset to increase turbulence inthe airstream of the air flow field 368.

Referring to FIG. 12, in some embodiments, openings 400 may be definedin the embossments 382 to connect the front side 378 and the back side380 in fluid communication. The openings 400 function much like theabove-described openings 226. The openings 400 may be circular holes,elongated slots, slits, or any other type of opening. The openings 400may be provided on the top 388, on the sidewalk 390, or combinationsthereof. The embossments 382 may include a single opening or a pluralityof openings. In some embodiments, the flow guide 350 may includeembossments that have openings and others that do not have openings.

Referring to FIG. 13, in another flow guide 410, the embossments mayproject from both the front and the back of the flow guide. For example,the flow guide 410 includes a first set of embossments 412 that projectfrom the front side 414 and a second set of embossments 416 that projectfrom the back. side. The embossments may be arranged in rows 41$ suchthat the first set 412 and the second set 414 of embossments alternatealong the length of the row. This creates a series of projections 420and depressions 422 along the length of the row. The exact arrangementof the first and second sets of embossments 412, 416 may vary tooptimize the flow guides for particular applications.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms encompassed by the claims.The words used in the specification are words of description rather thanlimitation, and it is understood that various changes can be madewithout departing from the spirit and scope of the disclosure. Aspreviously described, the features of various embodiments can becombined to form further embodiments of the invention that may not beexplicitly described or illustrated. While various embodiments couldhave been described as providing advantages or being preferred overother embodiments or prior art implementations with respect to one ormore desired characteristics, those of ordinary skill in the artrecognize that one or more features or characteristics can becompromised to achieve desired overall system at tributes, which dependon the specific application and implementation. These attributes caninclude, but are not limited to cost, strength, durability, life cyclecost, marketability, appearance, packaging, size, serviceability,weight, manufacturability, ease of assembly, etc. As such, embodimentsdescribed as less desirable than other embodiments or prior artimplementations with respect to one or more characteristics are notoutside the scope of the disclosure and can be desirable for particularapplications.

What is claimed is:
 1. A bipolar plate for a fuel cell comprising: ananode plate having hydrogen flow channels on a first side of the anodeplate and coolant channels on a second side of the anode plate; acathode plate having a first side disposed against the second side ofthe anode plate to cover the coolant channels and having a second sidedefining a recessed pocket configured to receive a stream of air; and aflow guide disposed in the pocket such that an inlet manifold is formedalong a first edge of the flow guide and an outlet manifold is formedalong a second edge of the flow guide, wherein the flow guide defineschannels extending from the inlet manifold to the outlet manifold, andwherein a plurality of openings is defined by through the flow guide. 2.The bipolar plate of claim 1, where a resting height of the flow guideis greater than a depth of the pocket so that the flow guide iscompressed when the bipolar plate is installed on the fuel cell.
 3. Thebipolar plate of claim 1, wherein the channels include concave andconvex channels, and wherein the openings connect between the concaveand convex channels.
 4. The bipolar plate of claim 3, wherein each ofthe concave channels includes opposing sidewalls and a land extendingbetween the sidewalls and disposed against a bottom of the pocket. 5.The bipolar plate of claim 3, wherein adjacent ones of the concave andconvex channels share a sidewall, and wherein the openings are definedin the sidewalls so that the adjacent ones of the concave and convexchannels are in fluid communication.
 6. The bipolar plate of claim 3,wherein the concave and convex channels are substantially perpendicularto the first and second edges.
 7. The bipolar plate of claim 1, whereina width of the pocket is larger than a width of the flow guide to formthe inlet and outlet manifolds.
 8. The bipolar plate of claim 1, whereinthe cathode plate is a carbon-based composite, and the flow guide ismetal.
 9. A bipolar plate for a fuel cell comprising: an anode sidehaving hydrogen channels; a cathode side defining a recessed pocket;coolant channels disposed between the anode side and the cathode side;at least one air port in fluid communication with the pocket; and a flowguide disposed in the pocket such that an inlet manifold is formed alonga first edge of the flow guide and an outlet manifold is formed along asecond edge of the flow guide, wherein the flow guide defines channelsextending from the inlet manifold to the outlet manifold, and wherein aplurality of openings is defined through the flow guide.
 10. The bipolarplate of claim 9, wherein a resting height of the flow guide is greaterthan a depth of the pocket so that the flow guide is compressed when thebipolar plate is installed on the fuel cell.
 11. The bipolar plate ofclaim 9, wherein the channels include concave and convex channels, andwherein the openings connect between the concave and convex channels.12. The bipolar plate of claim 11, wherein adjacent ones of the concaveand convex channels share a sidewall, and wherein the openings aredefined in the sidewalk so that the adjacent ones of the concave andconvex channels are in fluid communication.
 13. The bipolar plate ofclaim 9 further comprising: an anode plate including the anode side; anda cathode plate including the cathode side, wherein the anode plate andthe cathode plate are attached to each other.
 14. The bipolar plate ofclaim 9, wherein the cathode plate is a. carbon-based composite, andwherein the flow guide is metal.
 15. A fuel cell comprising: a pluralityof unit cells disposed in a stack, each unit cell including: a membraneelectrode assembly (MEA) having an anode and a cathode, a bipolar platehaving a cathode side defining a recessed pocket in fluid communicationwith an air port, an anode side, and coolant channels between thecathode and anode sides, wherein the bipolar plate is disposed againstthe MEA such that the cathode is disposed over the pocket, and a flowguide disposed in the pocket with a front side facing the MEA and a backside facing a bottom of the pocket, wherein the flow guide includes aplurality of embossments.
 16. The fuel cell of claim 15, wherein atleast one of the embossments defines an opening extending from the frontside to the back side.
 17. The fuel cell of claim 15, wherein theembossments are raised from the front side and recessed into the backside.
 18. The fuel cell of claim 17, wherein the flow guide furtherincludes a plurality of second embossments raised from the back side andrecessed into the front side.
 19. The fuel cell of claim 15, wherein thepocket includes opposing first and second sidewalls spaced apart by afirst distance, and wherein the flow guide includes opposing first andsecond edges spaced apart by a second distance that is shorter than thefirst distance to create an air inlet manifold defined between the firstsidewall and the first edge and an air outlet manifold defined betweenthe second sidewall and the second edge.
 20. The fuel cell of claim 15,wherein the bipolar plate is a carbon-based composite, and wherein theflow guide is metal.